1. Field of the Invention
The present invention relates to an electrolytic copper plating solution containing a specific sulfur-containing compound and a thiol-reactive compound, and to an electrolytic copper plating process using such an electrolytic copper plating solution. The present invention also relates to a method for controlling the electrolytic copper plating solution by using an amount of a specific decomposition product of the sulfur-containing compound as an index.
2. Description of the Related Art
In recent years, high density and thin model printed wring boards have been strongly desired for making the size of electronic instruments such as personal computers small and enhancing the performance thereof. One of the techniques to comply with such a request is to use a multi-layer printed wiring board prepared by a build up method that performs a sequential lamination of forming a pattern on every layer (i.e., a build-up printed wiring board).
In late years, a via-filling method has been developed, where the whole micro via hole (MVH) is filled with a conductive material to electrically connect between adjacent layers of the build-up printed wiring board. In this method, an effective area of the printed wiring board can be increased. Further, in comparison with a plating only on the inner wall of the MVH by the conventional method, the via-filling method can provide a sufficient electric connection even when the MVH has a smaller diameter so that it is effective to obtain a small sized and high-density printed wiring board.
Several methods have been proposed as via-filling methods including a method of filling a conductive paste into the MVH using a printing process, a method of selectively stacking electroless copper plating by only activating a conductive layer of the bottom of the MVH, and a method using an electrolytic copper plating.
The conductive paste is a mixture of copper and organic compounds so that its electric conductivity is lower than that of a metal copper. Thus, it is difficult to obtain a sufficient electrical connection when the MVH having a small diameter is used. It means that it is not an effective method for fabricating a small-sized and high-density printed wiring board. Furthermore, the filling by printing process requires that a viscous paste is filled in a closed-end pore of a small diameter. In this case, however, it is difficult to fill the pore without remaining any space because of the viscosity of the paste. In addition, the method of using the electroless copper plating is superior than the method using the conductive paste in that the MVH filler is a high conductive metal copper deposit. However, the depositing rate of a plated film is substantially slow, so that there is a problem in its productivity. In the case of using a typical high-speed electroless copper plating bath, the depositing rate of the plated film may be almost 3 μm/hr. If the inside of a typical BVH (100 μm in diameter and 100 μm in depth) is to be filled by the high-speed electroless plating solution, its productivity is very unfavorable because it takes 30 hours or over for completing the plating.
In the case of the electrolytic copper plating, on the other hand, a depositing rate of the plated film is high (i.e., 10 to 50 μm/hr), so that the considerable reduction of processing time can be attained. Therefore, an application of electrolytic copper plating to the MVH has been expected. In the case of depositing the copper over the inside of the MVH, it is required to fill the inside of the MVH with the copper without remaining any space. For that purpose, the depositing rate at the vicinity of the bottom of the closed-end pore should be higher than the depositing rate at the opening thereof. If the depositing rate at the vicinity of the bottom is equal to or slower than the depositing rate at the opening, the MVH will not be filled up or the opening will be closed before completing the filling of the inside of the MVH with copper plating. Thus, some space is left in the inside of the pore. In any case, it has few practical applications.
Electrolytic copper plating baths containing a specific sulfur-containing compound have conventionally been used for the purpose of increasing the deposition rate in the vicinity of the bottom of MVHs. For this purpose, a direct-current electrolysis that makes use of a soluble anode, such as a phosphorus-containing copper anode, has generally been used as an electrolysis condition. While this approach can ensure a good MVH-filling property just after initial make-up of electroplating bath, the electrolytic copper plating bath becomes unstable over time and, after a certain length of time, the electrolytic copper plating layer forms aggregation, which affects the appearance of the plating and makes the via-filling property unstable. The causes for this phenomenon have yet to be theoretically understood and no effective approach for controlling the electrolytic copper plating solution that alleviates the instability of the plating solutions containing the above-described specific compound has been proposed thus far. Nor has there been proposed an electrolytic copper plating solution that has alleviated the problem of instability.